tests(healthchecks) add test for success in stream active healthchecks
tests(healthchecks) add test for success in stream active healthchecks

This serves as a regression test for the problem in
https://github.com/Kong/lua-resty-healthcheck/issues/19

The new tests are marked as #flaky because they are timing-dependent,
so we we will not run them regularly in CI.

Kong/kong,Kong/kong,Mashape/kong,Kong/kong